Ischaemic heart disease
Anterior myocardial infarction
Primary fibrosis of the conducting system (Lenegre disease)
In left bundle branch block (LBBB) the left ventricle is not directly activated by impulses travelling through the left bundle branch. The right ventricle, however, is still activated as normal by the right bundle branch.
The left ventricle is activated by impulses travelling through the myocardium across the septum. As this occurs more slowly than conduction through the bundle of His the QRS complex becomes widened.
Normally the septum is activated from left to right, which produces small Q waves in the lateral leads. In the presence of LBBB, however, this septal activation is reversed, which eliminates these normal septal Q waves.
The right to left depolarization of the myocardium produces deep S waves in the right praecordial leads (V1-V3) and tall R waves in the lateral leads (I, V5 and V6). It also usually causes left axis deviation. As the ventricles are activated sequentially from right to left, rather than simultaneously, the R wave in the lateral leads is broad and notched (‘M’ shaped)
Secondary T wave changes are a normal finding in LBBB. T wave changes are classed as secondary if the T wave is upright when the terminal portion of the QRS complex is negative and the T wave is inverted when the terminal portion of the QRS complex is positive. Primary T wave changes occur when these rules are violated and are consistent with myocardial ischaemia.
The diagnosis of ST-elevation myocardial infarction can be made in the presence of LBBB by using the Sgarbossa ECG algorithm.
The diagnostic criteria for LBBB are:
Broad QRS complex (> 120 ms)
Dominant S wave in lead V1
Broad, monophasic R wave in lateral leads (I, AVL, V5 and V6)
Prolonged R wave peak time > 60 ms in left praecordial leads (V5-V6)
Absence of Q waves in lateral leads (I, V5 and V6)